Devices and methods for dynamic fixation of skeletal structure

ABSTRACT

The disclosed screw assemblies include a screw that attaches onto the bone, a housing member that connects and inter-locks the bone screw to the rod, and one or more locking members that permit immobilization of various components of the assembly relative to one another while still permitting some relative movement. The bone screws and bone screw assemblies described herein permit flexible stabilization of the spine.

REFERENCE TO PRIORITY DOCUMENT

This application is a continuation of co-pending U.S. patent applicationSer. No. 11/360,038, filed Feb. 21, 2006, which claims priority of thefollowing U.S. Provisional Patent Applications: (1) U.S. ProvisionalPatent Application Ser. No. 60/749,719, filed Dec. 12, 2005; (2) U.S.Provisional Patent Application Ser. No. 60/731,690, filed Oct. 31, 2005;and (3) U.S. Provisional Patent Application Ser. No. 60/654,602, filedFeb. 18, 2005. Priority of the aforementioned filing dates is herebyclaimed, and the disclosures of the patent applications are herebyincorporated by reference in their entirety.

BACKGROUND

The present disclosure relates to methods and devices that permitstabilization of the bony elements of the skeleton. The methods anddevices permit adjustment and maintenance of the spatial relationship(s)between neighboring bones. Depending on the specifics of the design, themotion between skeletal segments may be increased, reduced, returned toa normal physiology state or modulated in any desired manner.

Surgical reconstruction of the bony skeleton is a common procedure incurrent medical practice. Regardless of the anatomical region or thespecifics of the reconstructive procedure, many surgeons employ animplantable device that can adjust, align and maintain the spatialrelationship(s) between adjacent bones.

Whether from degenerative disease, traumatic disruption, infection orneoplastic invasion, alteration in the anatomical relationships betweenthe spinal vertebras can cause significant pain, deformity anddisability. Spinal disease is a major health problem in theindustrialized world and the surgical treatment of spinal pathology isan evolving discipline. The traditional surgical treatment of abnormalvertebral motion has been the complete immobilization and bony fusion ofthe involved spinal segment. An extensive array of surgical techniquesand implantable devices has been formulated to achieve completeimmobilization.

The growing experience with spinal fusion has shed light on thelong-term consequences of vertebral immobilization. It is now acceptedthat fusion of a specific spinal level will increase the load on, andthe rate of degeneration of, the spinal segments immediately above andbelow the fused level. As the number of spinal fusion operations haveincreased, so have the number of patients who require extension of theirfusion to the adjacent, degenerating levels. The second procedurenecessitates re-dissection through the prior, scarred operative fieldand carries significantly greater risk than the initial procedure whileproviding a reduced probability of pain relief. Further, extension ofthe fusion will increase the load on the motion segments that now lie ateither end of the fusion construct and will accelerate the rate ofdegeneration at those levels. Thus, spinal fusion begets additional,future fusion surgery.

In view of the proceeding, there is a growing recognition that segmentalspinal fusion and complete immobilization is an inadequate solution toabnormal spinal motion. Correction of the abnormal movement andpreservation of spinal mobility is a more intuitive and rationaltreatment option. It is appropriate to employ motion correction in theinitial treatment plan and reserve complete immobilization and fusionfor those patients with advanced motion abnormalities that can not becorrected.

SUMMARY

Disclosed are dynamic bone screws that permit correction and control ofthe movement between adjacent bones. The screws can be used pursuant toan implantation protocol that provides ease of use as well as a safe andfamiliar surgical approach. The bone screws and bone screw assembliesdescribed herein permit flexible stabilization of the spine.

Complete immobilization of the spinal segment is most commonlyaccomplished by screw fixation of the bony elements while the bone graftmatures into a solid fusion. In order to preserve motion, a bone graftis not used with the bone screw assemblies described herein. Inaddition, the disclosed bone screw assemblies are adapted to permitmovement of the spinal segments to which the assemblies are attached,while still providing stabilization of the spinal segments. Thus, thedisclosed bone screw assemblies are adapted to stabilize spinalsegments, but still move or articulate in response to the imposition ofstress caused by the relative displacement of the spinal segments.

Bone fixation is accomplished by the attachment of a first bone screwassembly to one bone and a second bone screw assembly to a second bone.The two screw assemblies are interconnected using a rigid rod so thatthe bone segments are immobilized relative to each other. The disclosedscrew assemblies include a screw that attaches onto the bone, a housingmember that connects and inter-locks the bone screw to the rod, and oneor more locking members that permit immobilization of various componentsof the assembly relative to one another while still permitting somerelative movement. In the devices disclosed herein, a rigidinterconnecting rod is preserved and various embodiments of a dynamicscrew assembly are shown.

In one embodiment, a dynamic screw assembly is created by removing ornot employing a locking element of the bone screw assembly. In this way,a housing member of the bone screw assembly serves to place the bonescrew in proximity to a rod but does not immobilize the bone screw andthe rod relative to one another.

In another embodiment, a head of the bone screw resides within a housingmember and, within defined limits, the spatial relationship between thescrew shaft and the housing member can vary. A saddle member resideswithin a segment of the housing member. A surface or element of thesaddle member movably engages a complimentary surface or element of thehousing member. This permits movement of the saddle member within thehousing member. Characteristics of movement are defined by thecharacteristics of complimentary surfaces or elements between thehousing member and the saddle member that permit relative movementtherebetween. An inner aspect of the saddle member accommodates a rodthat can be used to connect the assembly to other bone screw assemblies.A first locking nut permits immobilization of the rod relative to thesaddle member and a second locking nut is used to immobilize the bonescrew relative to the housing member. In this way, a dynamic screwassembly is formed by providing movement between the saddle member andthe housing member. Further, the addition of a third locking nut cantransform the dynamic screw assembly into a rigid one.

In another embodiment, the bone screw locks onto one segment of thehousing member and the rod attaches onto another segment of that member.The housing member contains a movable interconnection between the twoattachment points. In this way, a dynamic screw assembly is created thatprovides movement within the housing member.

In another embodiment, a secondary moving element or surface is placedbetween the head of the bone screw and the central housing member. Whenthe assembly is locked, both the rod and the secondary moving surfaceare immobilized relative to the housing member. However, the bone screwremains mobile within the confines of the secondary moving surface.Thus, a dynamic screw assembly is created that provides movement betweenthe bone screw and housing member.

In another embodiment, the bone screw contains a movable intermediatesegment between the screw shaft that engages the bone and the screw headthat lies within the central housing member. In this way, a dynamicscrew assembly is created that provides movement within the bone screwitself.

In another embodiment, a malleable member surrounds the rod and screwwithin the confines of a rigid housing. In this way, a dynamic screwassembly is created that provides relative movement between the rod andscrew head.

The bone screws described herein form a dynamic screw assembly whilemaintaining use of a rigid rod between different screw assemblies. Whiledescribe as separate embodiments, the various mechanisms may be used incombinations to produce additional screw assemblies that have specificdesired properties (such as an axis of rotation within a specifiedspatial location).

In one aspect, there is disclosed a bone fixation assembly, comprising:an elongate rod; a bone fixation member adapted to be secured to aspinal segment; a housing assembly having a first portion adapted to beremovably attached to the rod in a manner that immobilizes the rodrelative to the first portion, the housing assembly also having a secondportion adapted to be removably attached to the fixation member to placethe rod and the fixation member in proximity; wherein at least a portionof the bone fixation assembly can articulate while the rod isimmobilized relative to the housing assembly to permit the fixationmember to move from an initial orientation to a different orientationrelative to the rod in response to application of a load on the bonefixation assembly, and wherein the fixation member is automaticallyurged toward the initial configuration when the load is removed.

In another aspect, there is disclosed a bone fixation assembly,comprising: an elongate rod; a bone screw adapted to be secured to aspinal segment; a housing assembly that receives the rod and thatreceives the screw so as to place the rod in proximity to the screw; alocking member that couples to the housing assembly to immobilize therod relative to the housing assembly; wherein at least a portion of thebone fixation assembly can articulate to permit the rod and the screw tomove from an initial spatial relationship to a second spatialrelationship, and wherein at least a portion of the bone fixationassembly urges the rod and the screw back toward the initial spatialrelationship upon movement away from the initial spatial relationship.

In another aspect, there is disclosed a bone fixation assembly,comprising: a bone screw adapted to engage a spinal segment; a housingmovably coupled to the bone-engaging member, the housing being adaptedto seat a fixation rod; and a locking member adapted to mate to thehousing to lock the fixation rod in a fixed position relative to thehousing; wherein the bone fixation assembly permits at least limitedrelative displacement between the bone screw and the fixation rod whilethe fixation rod is immobilized relative to the housing, the bonefixation assembly being reformed from an initial configuration to adifferent configuration in response to an imposition of stress on thebone screw assembly, and automatically recovering toward the initialconfiguration when the stress is removed.

In another aspect, there is disclosed a bone fixation assemblycomprising: an elongate rod; a bone fixation member adapted to besecured to a spinal segment; and a housing assembly having: an innerhousing having a slot receives the rod in a manner that permitsimmobilization of the rod relative to inner housing; and an outerhousing having a seat that receives a head of the bone fixation membersuch that a shank of the bone fixation member extends outwardly from theouter housing; wherein the inner housing movably mounts within the outerhousing to place the rod and the fixation member in proximity such thatthe fixation member can move from an initial orientation to a secondorientation relative to the rod in response to application of a load onthe bone fixation assembly, and wherein the fixation member isautomatically urged toward the initial configuration when the load isremoved.

In another aspect, there is disclosed a bone fixation assembly,comprising: a spinal rod; a housing assembly having a channel adapted toreceive the spinal rod; a locking member adapted to immobilize the rodrelative to the housing; and a bone screw extending from the housingassembly and adapted to engage a spinal segment; wherein at least aportion of the bone fixation assembly includes an articulation regionthat elastically articulates to permit relative movement between the rodand the screw.

In another aspect, there is disclosed a bone fixation assembly,comprising: an elongate rod; a bone fixation member adapted to besecured to a spinal segment; a housing assembly including an outerhousing with a slot that receives the rod, the outer housing furtherincluding a seat, and wherein the housing assembly further includes aninner housing positioned in the seat, wherein the inner housing definesa socket in which a head of the bone fixation member is movablypositioned; and a locking member adapted to immobilize the rod and theinner housing relative to the outer housing while the head is movablewithin the socket.

In another aspect, there is disclosed a bone fixation assembly,comprising: an elongate rod; a bone fixation member adapted to besecured to a spinal segment; and a housing assembly including an outerhousing and at least one locking member positioned within the outerhousing and adapted to receive the rod, the housing assembly furthercomprising a rotational member rotatably positioned within the lockingmember and the outer housing, the rotational member forming a socket inwhich a head of the fixation member is rotatably positioned; wherein therod can be pressed downward into the locking member to immobilize therod in the locking member and the locking member in the housing andfurther immobilize the rotational member relative to the locking memberwhile permitting rotatable movement of the head of the fixation memberwithin the socket.

In another aspect, there is disclosed a method of stabilizing the spine,comprising: providing a fixation assembly including a housing, a bonescrew, and a rod; securing the fixation assembly to a spine segment suchthat the bone screw is fixated within the spine segment; and locking thefixation assembly such that the rod is immobilized relative to thehousing while the bone screw can move from an initial orientation to adifferent orientation relative to the rod in response to application ofa load on the bone fixation assembly, and wherein the bone screw isautomatically urged toward the initial configuration when the load isremoved

These and other features will become more apparent from the followingdescription and certain modifications thereof when taken with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a conventional, rigid fixation screw assembly.

FIG. 2 shows a cross-sectional view of the assembly in a lockedposition.

FIGS. 3A and 3B show an exploded view and a cross-sectional assembledview of another embodiment of a non-rigid bone screw assembly.

FIG. 4 shows an assembled view of another embodiment of a bone screwassembly that permits movement of the screw, rod, and/or housingrelative to one another prior to complete locking of the device.

FIG. 5 shows an exploded view of the assembly of FIG. 4.

FIG. 6 shows a perspective view of a bone screw of the assembly of FIGS.4 and 5.

FIG. 7A shows a perspective view of an outer housing of the assembly ofFIGS. 4 and 5.

FIG. 7B shows a cross-sectional, perspective view of the outer housing.

FIG. 8A shows a perspective view of a locking member of the assembly ofFIGS. 4 and 5.

FIG. 8B shows a cross-sectional view of the locking member.

FIGS. 9 and 10 show a perspective views of a saddle member of theassembly of FIGS. 4 and 5.

FIG. 11 shows a cross-sectional view of the saddle member.

FIG. 12 shows a perspective view of the assembly in a partiallyassembled state.

FIG. 13 shows a cross-sectional view of the assembly in the partiallyassembled state.

FIG. 14 shows a perspective view of the assembly with the saddle memberbeing inserted into the outer housing.

FIG. 15 shows the assembly with the saddle member fully inserted intothe outer housing.

FIG. 16 shows a cross-sectional view of the assembly.

FIG. 17 shows the assembly with the rod positioned within the saddlemember and the inner locking nut positioned to immobilize the rod inplace relative to the saddle member.

FIG. 18A shows the assembly with the outer locking nut engaged with theouter housing.

FIG. 18B shows a cross-sectional view of the assembly with the innersaddle member positioned within the outer housing.

FIG. 19 shows the assembly with the central locking nut engaged withinan outer locking nut and the assembly fully assembled.

FIGS. 20-22 show vertebral bodies V1 and V2 with a pair of bone screwassemblies attached to each vertebral body.

FIG. 23 shows another embodiment of the dynamic bone screw assembly in afully assembled state.

FIG. 24 shows the assembly of FIG. 24 in an exploded state.

FIG. 25 shows a cross-sectional view of the assembly of FIG. 23.

FIG. 26 shows another embodiment of a bone screw assembly.

FIG. 27 shows a cross-sectional view of the assembly of FIG. 26.

FIGS. 28A-28B show side and cross-sectional views of an additionalembodiment of a bone screw

FIG. 29 shows yet another embodiment of a bone screw assembly.

FIG. 30 shows an exploded view of the assembly of FIG. 28.

FIG. 31 shows a cross-sectional view of the assembly of FIG. 28.

FIG. 32 shows another embodiment of a dynamic bone screw assembly.

FIG. 33 shows the bone screw assembly of FIG. 32 in an exploded state.

FIG. 34 shows a cross-sectional view of the assembly of FIG. 32.

FIG. 35 shows how the screw of the assembly can rotate to variouspositions.

FIG. 36 shows the screw assembly of FIG. 32 placed into the pedicleportion of a vertebra.

FIG. 37A shows an exploded view of another embodiment of a dynamic bonescrew assembly.

FIG. 37B shows a cross-sectional view of the bone screw assembly of FIG.37A.

FIG. 37C shows an exploded view of another embodiment of a dynamic bonescrew assembly.

FIG. 37D shows a cross-sectional view of the bone screw assembly of FIG.37C.

FIG. 38 show various schematic embodiments of bone screw assemblies thatemploy multiple moving surfaces or elements.

DETAILED DESCRIPTION

Disclosed are methods and devices for fixating a device to a skeletalstructure, such as to a spinal motion segment (also referred to as aspinal segment), such as a vertebra. The disclosed bone screw assembliesare adapted to secure an elongate rod (or any other type of elongateelement or stabilizer for use in conjunction with a spinal fixationsystem) to a bone fixation device, such as a bone screw or a bone hookthat attaches to a spinal segment. The disclosed bone screw assembliesare adapted to secure the rod to the fixation device while permitting atleast some movement or articulation of the rod relative to the fixationdevice, such as in response to the imposition of stress caused by therelative displacement of the attached spinal segments. The assembliescan include one or more locking members that can be actuated or adjustedto vary the level and/or type of relative movement between the rod andthe fixation device and to also completely immobilize the rod relativeto the fixation device if such immobilization is desired. The assembliescan be partially or entirely manufactured of one or more materials thatenable such relative movement, as described below.

The disclosed dynamic assemblies permit spine stabilization in that afirst spinal segment can be attached to a second spinal segment usingrod-interconnected bone screw assemblies, wherein each assembly isattached to its respective spinal segments via a bone fastener. The bonescrew assemblies can include at least one dynamic bone screw assembly ofthe type described herein. Bone screws are the most commonly usedfasteners in spinal stabilization and the devices disclosed in thisapplication will illustrate dynamic assemblies that utilize bone screwfasteners. However, it is understood that hooks, clamps, cables, or anysuitable fastener may be alternatively used. The stabilization isflexible in that each assembly permits at least some relative movementbetween the rod and the screw thereby permits the spinal segment to moverelative to the rod. The interconnected bone screw assemblies permit atleast some control over relative movement between the vertebralsegments, while permitting movement of the spinal segments relative toone another.

The screw assemblies described herein can vary in structure and itshould be appreciated that the disclosure is not limited to theparticular structures described herein. Some embodiments include ahousing that couples to a rod or other type of longitudinal element foruse in conjunction with a spinal fixation system. One or more lockingmembers are used to immobilize the housing relative to the rod. Thehousing also couples to a bone screw that fixedly attaches to a spinalsegment. One or more locking members are used to couple the bone screwrelative to the housing. The structural arrangement and/or materials ofmanufacture of the components of the screw assemblies are adapted topermit relative movement between the rod and the screw, between the rodand the housing, between the screw and the housing, within the screw,and combinations thereof.

In some implementations, the housing can be manufactured of two or morecomponents that attach to one another in a manner that permits limited,relative movement between the housing components. The rod can be fixedlyattached to one or more of the components while the screw is fixedlyattached to one or more separate components of the housing. Because thecomponents can move relative to one another, the rod and the screw canmove relative to one another while still being commonly attached to thehousing. The screw, housing, and/or intermediate components can also bemanufactured of a material that is deformable or flexible to permitrelative movement via deformation of the components themselves.

In this regard, the components of the screw assembly can be entirely orpartially manufactured of a shape-memory material that exhibitssuperelastic (also known as pseudoelastic) characteristics. Shape memorymaterials (typically shape memory alloys (SMAs)) are materials that canbe deformed at one temperature, but when heated or cooled, the materialsreturn to their original, pre-deformed shape. Thus, the material“memorizes” a previous shape

Shape memory materials undergo a reversible transformation from onecrystal phase to another over a particular temperature range. Above thistemperature range, the material exists as austenite, which has a rigidcrystal structure. The shape of a component while in the austenite phaseis typically-referred to as the memory shape. A low temperature phase,martensite, is soft and can be deformed from its original shape withoutcausing any permanent deformation. Once deformed, martensitic materialwill remain in this deformed shape indefinitely. When heated later, thematerial transforms to the high temperature phase and returns to itsmemory shape. The transformation between martensitic and austeniticphases can occur as a result of a change of temperature or as a resultof the imposition of stress on the material. In this regard, athermoelastic martensitic transformation has occurred if thetransformation occurs in response to a change in temperature. If themartensitic transformation occurs as a result of the imposition ofstress, then a stress-induced martensitic transformation has occurred.

Shape memory materials can exhibit superelasticity wherein a small forceinduces considerable deformation but when the force is removed, thematerial automatically recovers its original shape without the need forheating. The superelastic phenomena occurs when stress is applied to ashape memory material at a temperature slightly higher than thetemperature at which the material begins to transform into austenite.When stressed, the material first deforms elastically up to the yieldpoint of the material. When further stress is applied to the material,it begins to transform into stress-induced martensite. When the stressis removed, the material transforms back into austenite and the materialreturns to its original, memorized shape.

A nickel-titanium alloy know as Nitinol is an example of a shape-memorymaterial. Nitinol is advantageous for use in the screw assembliesdescribed herein, as Nitinol can be programmed to undergo astress-induced martensitic transformation at about normal human bodytemperature (i.e., at about 35-40 degrees Celsius).

An article entitled Shape Memory Effect and Super-Elasticity in Ni—TiAlloys, Titanium and Zirconium, Vol. 30, No. 4, October 1982 (which isincorporated herein by reference), by Yuichi Suzuki, provides detailsregarding the superelasticity. The disclosed screw assemblies can be atleast partially manufactured of a shape memory material that exhibitssuperelastic characteristics or behavior at about human bodytemperature.

FIG. 1 shows a conventional, rigid fixation screw assembly. FIG. 2 showsa cross-sectional view of the assembly 105 in a locked position. Theassembly 105 includes a fixation member comprised of a bone screw 110,an elongate rod 115, and a housing 120 that couples to both the bonescrew 110 and the rod 115. The housing 120 has a slot that is sized toreceive the rod 115. The assembly of FIGS. 1 and 2 is rigid in that thebone screw is completely immobilized relative to the housing 120 and therod 115 when a locking member is tightened onto the rod. Thus, norelative movement between the rod and the bone screw is permitted whenthe locking member is tightened onto the rod.

An internal bore in the housing 120 is sized to receive the screw 110.In this regard, a head 122 of the bone screw 110 sits within a seat inthe housing 120 such that an anchoring portion (such as a shank portion124 or a hook) of the bone screw 110 protrudes downwardly out of thehousing 120. A locking member 125 sits above the head 122 and below therod 115 in the housing 120. A locking nut 130 can be advanced downwardinto the housing 120 to force the rod 115 downwardly against the lockingmember 125 and compress the screw 110 against the inner aspect of thehousing 110. When locking nut 130 is fully advanced, the assemblybecomes rigid such that the screw 110 is completely immobilized relativeto the housing 120 and the rod 115.

There are now described various embodiments of bone screw assembliesthat are non-rigid, such that there is at least some level of relativemovement between the rod 115, the housing 120 and/or the bone screw 110while still placing the rod in proximity to the bone screw. Theassemblies can also be rigid upon the actuation of locking members.

In one embodiment, one or more of the components of the assembly ofFIGS. 1 and 2 are manufactured of a deformable or flexible material. Thematerial can be a shape memory material, for example. Because thematerial is deformable, at least some level of movement of thecomponents relative to one another is enabled. For example, the saddle125 can be manufactured of a material that deforms upon the satisfactionof deformation criteria, such as upon the application of a thresholdlevel force. If such a threshold level of force is applied to the saddle122 via the screw, then the screw can be rotated relative to the housing120. Other components of the assembly 105 can also be manufactured of adeformable material.

Thus, the components of the assemblies described herein can bemanufactured of a shape memory material. The material has a memorizedshape wherein the screw is placed in a first orientation relative to therod. Upon the imposition of stress or a load to the material, thematerial transforms to a different shape that places the screw in adifferent orientation relative to the rod. If the material is reshapedor deformed while at a temperature above the material's transformationtemperature, the material automatically recovers toward its memorizedshape when the stress is removed. In one embodiment, a screw assembly isattached to a spinal segment while at least one component (such as thehousing, rod, screw, or portion thereof) of the assembly is in asubstantially unstressed initial configuration where virtually all ofthe shape memory material is in an austenitic state. Upon the impositionof stress onto a portion of the assembly (which can be caused byrelative movement between the spinal segments), at least a portion ofthe material is transformed into reversible stress-induced martensite.Upon the reduction or removal of stress, at least a portion of thematerial is transformed back into austenite.

Any of the components of the bone screw assemblies described herein canbe at least partially formed of a shape-memory material that exhibitspseudoelastic characteristics or behavior at about human bodytemperature. As mentioned, Nitinol is an example of a material that canbe programmed to undergo a stress-induced martensitic transformation atabout normal human body temperature (i.e., at about 35-40 degreesCelsius). It should be appreciated that any of the embodiments describedherein can be at least partially manufactured of a deformable material.

FIGS. 3A and 3B show an exploded view and a cross-sectional assembledview of another embodiment of a non-rigid bone screw assembly 305. Theassembly 305 includes a bone screw 310, a housing 320, a saddle 325, anda locking nut 330. The saddle 325 is interposed between a head of thescrew 310 and the rod when the assembly 305 is assembled. For clarity ofillustration, the rod is not shown in FIGS. 3A and 3B, although the rodis adapted to be received within a slot 332 in the housing 320.

The saddle 325 is dimensioned such that the rod 315 does not directlycompress against the saddle 325 when the rod is pressed fully downwardinto the slot 332 in the housing 320. Rather, the rod 315 abuts thebottom edge of the slot 332. The bottom edge is above the level of thetop surface of the saddle 325 such that the rod 315 does not pressdownward against the saddle 325 when the rod is pressing against thebottom edge of the slot 332. Full advancement of the locking nut 330locks or immobilizes the rod 315 relative to the housing 320, but stillpermits movement of the screw 310 relative to the housing 320. That is,the head 322 of the screw 310 can rotate within the housing 310 when thelocking nut 330 is fully advanced downward against the rod 115. When thehead 322 rotates, the orientation of the longitudinal axis of the screw310 varies.

With reference to FIG. 3B, a space 335 is located between the head 322of the screw 310 and the rod when the rod is positioned in the housing320. The space 335 can be loaded with a material that resists movementof the screw, but still permits some movement when a load of sufficientforce is applied to the screw. Thus, the space can be fitted, forexample, with springs, Belleville washers, fluids, elastic materials,magnets or any know mechanism that can be adapted to resist movement ofthe screw 310 within the housing 320. In this manner, the screw 310 canmove relative to the housing 310, but only if a force is applied to thescrew wherein the force is of sufficient magnitude to overcome themovement-resistant material or structure within the space 335.

This screw assembly has a neutral position wherein longitudinal axis ofthe housing is perpendicular to the plane atop the head of the screw, asshown in FIG. 3B. In the neutral position, the net force acting upon thescrew is zero. However, when the screw is moved outside of the neutralposition (such that the plane atop of the screw head is no longerperpendicular or substantially perpendicular to the longitudinal axis ofthe housing), the material placed in space 335 will exert a net force onthe head of the screw and return the screw to the neutral position. Inthis embodiment, the neutral position is pre-determined. That is, thereis a pre-determined relationship between the longitudinal axis of thehousing and plane atop the screw head (such as perpendicular). Thatrelationship is a function of screw design and cannot be changed by thesurgeon at the time of screw placement.

At implantation, the screw assembly of the current embodiment will be inthe neutral position. In order to connect several non-linear screwassemblies with a single rod, the housing of one or more assemblies mustbe taken out of the neutral position. This maneuver will necessarilycause the bone screws of the assemblies in the non-neutral position toapply significant load onto the attached bones. Since it is sometimesundesirable to place a load on the vertebral bones at the time of screwimplantation, other embodiments are illustrated that will obviate thisfeature. In those embodiments, the housing and bone screw may be placedin any desired position relative to one another before the assembly'sneutral position is set. That is, the neutral position is notpre-determined in those embodiments.

FIG. 4 shows an assembled view of another embodiment of a bone screwassembly 400 that permits movement of the screw, rod, and/or housingrelative to one another prior to complete locking of the device. FIG. 5shows an exploded view of the assembly of FIG. 4. The assembly of FIGS.4 and 5 includes a housing that is formed of several components that canmove or articulate relative to one another. The rod can be immobilizedrelative to a first component while the screw can be immobilizedrelative to a second component of the housing. Because the first andsecond components are movable relative to one another, the rod and screwcan move relative to one another while still being coupled to oneanother.

The assembly includes a housing comprised of an outer housing 405 and aninner saddle member 410 having a slot 412 for receiving a rod 415 (FIG.5). A locking member 420 (FIG. 5) fits within the outer housing 405above a bone screw 425. The bone screw 425 sits within a seat in thebottom of the outer housing 405 such that a shank of the screw 425extends outwardly from the outer housing 405. An inner locking nut 430interfaces with the saddle member 410 for providing a downward load onthe rod 415 for securing the rod relative to the saddle member 410, asdescribed below. An outer locking nut 435 interfaces with the outerhousing 405 for locking the assembly together, as described below. Acentral locking nut 440 engages a central, threaded bore within theouter locking nut 435. The locking nuts 430, 435, and 440 can providevarious combinations of immobilization of the rod 115, screw 425, andhousing relative to one another.

FIG. 6 shows a perspective view of the bone screw 425. The bone screw425 includes a shank 605 that extends from a head 610. The head 610 hasan upper surface in which is disposed a drive connector such as a cavity620 that is sized and shaped to receive a tool for driving the screw 425into bone. The cavity 620 can be, for example, hexagonal shaped toreceive a hex drive for engaging and rotating the screw 425.

FIG. 7A shows a perspective view of the outer housing 405. FIG. 7B showsa cross-sectional, perspective view of the outer housing 405. A centralbore extends through the outer housing for receipt of the bone screw425. A seat 705 is located within a base region 706 of the outer housing405 for receiving the head 610 of the screw 425. The base region 706 hasa pair of upper surfaces 708 that face a region of the saddle member 410in the assembled device. The upper surfaces 708 can be convex along twodimensions or can have any contour.

A pair of opposed extensions 710 extend upwardly from the base region706 and flare outwardly to form into threaded regions 715 that interfacewith the outer locking nut 435 (FIG. 5). Each of the extensions 710 hasan inner surface that includes an elongate slot 720 that slidinglyengages a complementary-shaped extension of the locking member 420, asdescribed in detail below.

FIG. 8A shows a perspective view of the locking member 420 of theassembly of FIGS. 4 and 5. FIG. 8B shows a cross-sectional view of thelocking member 420. As mentioned, the locking member 420 is positionedwithin the outer housing 425 above the screw 425 in the assembleddevice. A base 805 has a hole 810 extending therethrough wherein thehole 810 is located directly above the drive cavity 620 of the bonescrew 425 when the device is assembled. As shown in the cross-sectionalview of FIG. 8B, a bottom region of the base 805 forms a cavity 807 thatis positioned immediately above the head 610 of the screw 524 in theassembled device.

A pair of opposed extensions 815 extend upwardly from the base 805. Eachextension has a rail 820 positioned on an outer surface of theextension. The extensions 815 are positioned relative to one anothersuch that they can fit in-between the extensions 710 (FIGS. 7A and 7B)of the outer housing 405. In addition, the rails 820 are sized, shapedand positioned to slidingly engage the slots 720 (FIGS. 7A and 7B) ofthe extensions 710 of the outer housing 405, as described in more detailbelow.

FIGS. 9 and 10 show a perspective views of the saddle member 410. FIG.11 shows a cross-sectional view of the saddle member 410. The saddlemember 410 has a pair of extensions 905 that form a rod channel 910therebetween wherein the channel 910 is adapted to receive the rod 415.A threaded engagement region 915 on the inner surface of the extensions905 is adapted to interface with the inner locking nut 430 (FIG. 5). Theouter aspect of each extension 905 includes a pair of protrusions 920that function to limit the amount of movement of the saddle 410 relativeto the outer housing 405 of the assembled device, as described in detailbelow. As best shown in FIGS. 9 and 11, a borehole 925 extends through abase of the saddle member 410.

With reference to FIGS. 9-11, the saddle member 410 has a bottom surface935 that is positioned adjacent to the upper surfaces 708 (FIGS. 7A, 7B)of the outer housing 405 in the assembled device. The bottom surface 935can have a contour that is selected to permit relative movement of thesaddle member 410 and the outer housing 405 such that the bottom surface935 can slide relative to the upper surfaces 708, as described below.For example, the bottom surface 935 can be concave along two dimensions.The saddle member 410 is dimensioned to fit within the outer housing405. In this regard, the saddle member 410 is at least slightlyundersized relative to the space between the extensions 710 of the outerhousing 405 to permit some travel or movement between the saddle memberand the outer housing in the assembled device.

FIG. 12 shows a perspective view of the assembly 400 in a partiallyassembled state with the screw 425 and the locking member 420 engagedwith the outer housing 405. FIG. 13 shows a cross-sectional view of theassembly 400 in the partially assembled state. The head 610 of the screw425 is positioned within the seat in the base region 706 of the outerhousing 405 such that the shank 605 extends through the bore in theouter housing 405. The head 610 is free to move within the seat. Thatis, the head 610 can rotate within the seat in a ball and socket manner.

With reference to FIGS. 12 and 13, the locking member 420 is positionedwithin the outer housing 405 such that the rails 820 are slidablepositioned within the slots 720 of the extensions 710 on the outerhousing 405. The extensions 815 on the locking member 420 have a heightsuch that upper edges of the extensions 815 extend past the upper edgesof the extensions 710 of the outer housing 405. In this manner, theupper edges of the extensions 815 can be pressed downwardly so that thelocking member 420 exerts a locking force on the head 610 of the screw425 to immobilize the screw 425 relative to the outer housing 405, asdescribed in detail below. The outer locking nut 435 can be used topress the upper edges of the extensions 815, as described below. Theborehole 810 (FIG. 12) is positioned above the drive cavity 620 in thehead 610 of the screw 425 to permit a hex drive to be engaged with thedrive cavity 620.

FIG. 14 shows a perspective view of the assembly with the inner saddlemember 410 deviated to one side within housing 405. FIG. 15 shows theassembly with the saddle member 410 in the midline (“neutral”) positionwithin outer housing 405. FIG. 16 shows a cross-sectional view of theassembly. The saddle member 410 slides into the space between theextensions 710 on the outer housing 405 and the locking member 420. Asbest shown in FIG. 16, the upper edges of the extensions 905 on thesaddle member 410 are positioned below the lower edges of the widenedthreaded region 715 of the outer housing 405 with a small space Bpositioned therebetween. A small space B is positioned between the lowersurface of the saddle member 410 and the surface 708 of the outerhousing 405.

With reference to FIG. 16, a space A exists between the sides of thesaddle member 410 and the inner sides of the outer housing 405 andlocking member 420. With reference to FIG. 15, a space 1505 is locatedbetween the protrusions 920 and the extensions 710. The size of thespace is limited by the size of the protrusions 920. The spaces A, B,and 1505 permit the saddle member 410 to have some play or movementrelative to the outer housing 405 when the saddle member 410 ispositioned in the outer housing 405.

It should be appreciated that the size and shape of the spaces can bevaried. Moreover, the saddle member 410 can be sized and shaped relativeto the outer housing 405 such that other spaces are formed. At least onepurpose of the spaces is to permit relative movement between the saddlemember 410 and the outer housing 405 and this can be accomplished invarious manners. Thus, the screw can be moved from a first orientation(such as the neutral position) to a second orientation while the rod isimmobilized relative to the inner member 410.

Any of the spaces, A, B, or 1505 can be fitted with an elastic ordeformable material or other mechanism, such as a spring, that resistssuch movement of the saddle member 410 within the outer housing 405. Inthis way, the device will resist movement to either side and will returnto a predetermined position, such as a mid-line position, after anapplied force has dissipated.

FIG. 17 shows the assembly 400 with the rod 415 positioned within thesaddle member 410 and the inner locking nut 430 positioned to immobilizethe rod 415 in place relative to the saddle member 410. As mentioned,the rod 415 sits within the channel 910 of the saddle member 410. Theinner locking nut 430 can be threaded downwardly into the saddle member410 so as to provided a downward force on the rod 415 and lock the rod415 relative to the saddle member. When fully seated, the inner lockingnut 430 locks the rod 415 within inner saddle member 410. At the stageshown in FIG. 17, the rod 415 is immobilized relative to the saddlemember 410, while the screw 425 can still rotate within the seat in theouter housing 405. Thus, both the screw and the rod are attached to theouter housing but the screw and the rod can have relative movement withrespect to one another.

FIG. 18A shows the assembly 400 with the outer locking nut 435 engagedwith the outer housing 405. The outer locking nut 435 has internalthreads that engage the threaded region 715 (FIG. 7A) of the outerhousing 405. The outer locking nut 435 can be threaded downward onto theouter housing 405. As this occurs, the outer locking nut 435 provides adownward force on the upper edge of the extensions 815 (FIG. 13) of thelocking member 420. As mentioned, the upper edges of the locking memberextend upwardly past the upper edges of the outer housing 405. The outerlocking nut 435 thus presses the locking member 420 downward, which inturn presses downward on the head of the screw 425. The head of thescrew 425 is pressed downward into the seat 705 (FIGS. 7A and 7B) of thehousing 405 with a force sufficient to immobilize the screw 425 withinthe seat of the outer housing.

At this stage, the bone screw 425 is immobilized relative to the outerhousing 405 due the outer locking nut 435 and the locking member 420pressing downward on the screw head. The inter-connecting rod 115 islocked or immobilized relative to the inner saddle member 410 due to thedownward force provided by the inner locking member 430 (FIG. 17).However, the inner saddle member 410 can move relative to the outerhousing 405 due to the spaces A and B (FIG. 16) and the space 1505 (FIG.15) between the inner saddle member 410 and the outer housing 405. Thus,the rod 115 can move relative to the screw 425 while both components arestill coupled to the housing.

With reference to FIG. 15, the inner saddle member 410 can slidably movewithin the outer housing 405 along a direction aligned with axis Swherein the amount movement is limited by the interplay between theprotrusions 920 and the extensions 710. This type of movement isrepresented in FIG. 18B, which shows a cross-sectional view of theassembly with the inner saddle member 410 positioned within the outerhousing 405. The inner saddle member 410 is represented in solid linesat a first position and in phantom lines at a second position aftersliding from right to left in FIG. 18B. The bottom surface 935 of theinner saddle member 410 slides along the upper surface 708 of the outerhousing 405. As mentioned, the surfaces can be contoured such that theinner saddle member slides along an axis S that has a predeterminedradius of curvature. This can be advantageous during flexion andextension of the attached spinal segments, as the radius of curvature ofthe axis S can be selected to provide motion along the physiologic axisof rotation of the spinal segments.

With reference now to FIG. 16, the inner saddle member 410 can also moveup and down along axis U and side-to-side along axis R relative to theouter housing 405 due to the spaces A and B. Some rotational and/orpivoting movement of the inner saddle member 1505 relative to the outerhousing 405 is also possible. Thus, movement of the inner saddle memberrelative to the outer housing provides the dynamic quality of the deviceas such movement permits the rod to move relative to the screw.

When complete immobilization is desired, the central locking nut 440 isadvanced into a threaded bore in the outer locking nut 435. The centrallocking nut 440 presses downward against the upper surfaces 1605 (FIG.16) of the inner saddle member 410 to force the inner saddle memberdownward against the outer housing 405. The inner saddle member isthereby immobilized relative to the outer housing. In this way, a fixedscrew configuration is produced. FIG. 19 shows the assembly 400 with thecentral locking nut 440 engaged within the outer locking nut 435 and theassembly fully assembled.

The assembled device or any of its components can be made of anybiologically adaptable or compatible materials. Materials consideredacceptable for biological implantation are well known and include, butare not limited to, stainless steel, titanium, tantalum, combinationmetallic alloys, various plastics, resins, ceramics, biologicallyabsorbable materials and the like. Any components may be alsocoated/made with osteo-conductive (such as deminerized bone matrix,hydroxyapatite, and the like) and/or osteo-inductive (such asTransforming Growth Factor “TGF-B,” Platelet-Derived Growth Factor“PDGF,” Bone-Morphogenic Protein “BMP,” and the like) bio-activematerials that promote bone formation. Further, the outer surface of thebone screw 425 may be made with a porous ingrowth surface (such astitanium wire mesh, plasma-sprayed titanium, tantalum, porous CoCr, andthe like), provided with a bioactive coating, made using tantalum,and/or helical rosette carbon nanotubes (or other carbon nanotube-basedcoating) in order to promote bone in-growth or establish a mineralizedconnection between the bone and the implant, and reduce the likelihoodof implant loosening. As discussed above, the assembly or its componentscan also be entirely or partially made of a shape memory material orother deformable material.

A placement and implantation protocol for the assembly 400 is nowdescribed. With the assembly 400 in the partially assembled stated shownin FIGS. 15 and 16, a hex-drive screw driver is used to engage thehex-shaped drive cavity 620 (FIG. 6) within the head 610 of the bonescrew 425. The driver transverses the bore 925 (FIG. 9) of the innersaddle member 410 and the bore 810 (FIG. 8A) of the locking member 420to reach the bone screw 425. The screw 425 is then rotated and anchoredinto the underlying bone. The shank portion 605 of the screw 425 engagesthe bone such that the screw 425 is locked to the bone.

When used in the spine, the screw 425 can be placed into the pediclesegment of the vertebra. A screw 425 of a second assembly 400 is placedinto a second vertebral body on the same side of midline as the firstscrew. The inter-connecting rod 415 is seated into the rod channel 910of each inner saddle member 410 such that the rod 415 connects the twoassemblies 400 and the respective screws 425. The inner locking nut 430is used to engage the threads 915 of the inner saddle member 410, butthe inner locking nut 430 is not fully tightened. At this point, the rod415 remains mobile within each rod channel of the saddle members 410.

The outer locking nut 435 is placed and used to engage the threads onthe top region of the outer housing 405. The nut 435 is locked therebyimmobilizing the bone screw 425 within the outer housing 405. Ifvertebral re-alignment is desired, the screws 425 are used tore-position the attached vertebral bodies. Since the inner locking nut430 is not yet locked, the rod 415 can slide within the channels of thesaddle members 410 and the screws 425 are still free to move relative toone another.

After the screws 425 are appropriately positioned, the inner lockingnuts 430 are fully advanced against the rod 415 to lock the rod 415relative to the inner saddles 415. FIG. 20 shows vertebral bodies V1 andV2 with a pair of assemblies 400 attached to each vertebral body. Twoscrews 425 can be placed into each vertebral body as shown in FIG. 20.FIGS. 20 and 21 show the vertebral bodies V1 and V2 in flexion and therelative movement permitted by the dynamic screws assemblies 400. Notethat each inner saddle member 410 can move within each outer housing405. FIG. 22 shows the vertebral bodies V1 and V2 in extension. Aspreviously discussed, the spaces 1505 (FIG. 15) may be fitted withelastic materials, springs, magnets, or any other device that can resistmovement of the saddle member 410 relative to outer housing 405. Thisfeature would enable the screw 425 to return the vertebral bodies to theneutral position after movement. Since the saddle member 410 wasslightly undersized in width, a limited amount of vertebral rotation isalso permitted. Further, before locking the assembly, the surgeon canfreely adjust the orientation of the screw relative to housing withoutinfluencing the assembly's neutral position or pre-loading the screw andbone construct.

In one embodiment, a rigid bone screw assembly is attached to a firstspinal segment or to any bone structure. The rigid bone screw assemblyis configured to completely immobilize a rod relative to a first bonescrew that is attached to the spinal segment such that the rod isfixedly cantilevered from the bone screw assembly. The rod is thencoupled to a dynamic bone screw assembly of the type described hereinsuch that the rod is placed in proximity to a second bone screw of thedynamic bone screw assembly. The dynamic bone screw assembly permitssome movement of the rod relative to the second bone screw. The dynamicbone screw assembly can permit up and down and/or rotational movementbetween the rod and the second bone screw while prohibitingtranslational movement. Thus, a rod can be coupled to a rigid bone screwassembly and to a dynamic bone screw assembly wherein each bone screwassembly is attached to a respective spinal segment.

FIG. 23 shows another embodiment of the dynamic bone screw assembly in afully assembled state. FIG. 24 shows the assembly of FIG. 24 in anexploded state. The assembly 2300 includes a housing 2305 that receivesa bone screw 2310 through an inner bore in the housing 2305. The housing2305 includes a slot 2317 that receives a rod 2315. A first locking nut2320 can be used to lock the bone screw 2310 relative to the housing2305 by providing a downward force against the head of the bone screw2310 that immobilizes the bone screw within a seat inside the housing,2305. Likewise, a second locking nut 2325 can be used to lock the rod2315 relative to the housing 2305 by pressing the rod 2315 downwardagainst a bottom surface of the slot 2317. The housing 2305 includes aflexible or articulating region 2330 that is configured to enable afirst region of the housing 2305 to move relative to a second region ofthe housing 2305, as described below.

FIG. 25 shows a cross-sectional view of the assembly of FIG. 23. Thescrew 2310 has a shank 2505 that extends from a head 2510. The head 2510sits within a seat formed within the bottom region of the housing 2305.The first locking nut 2320 has threads that engage corresponding threadsinside the housing 2305. The first locking nut 2320 can be advanceddownward to exert a force on the head 2510 of the screw 2310 to therebyimmobilize or lock the screw 2310 relative to the housing 2305. Thescrew head and/or the seat in which it sits may be serrated, textured,coated, corrugated or otherwise treated in any manner intended toincrease the frictional forces between them so as to potentate thelocking mechanism. This feature may be equally applied to any otherembodiment disclosed in this application.

The rod 2315 sits within the channel 2317 in the housing 2305. Thesecond locking nut 2325 engages a threaded region in the housing 2305and can be advanced downward against the rod 2215. The second lockingnut 2325 provides a downward force to press the rod 2315 against thebottom of the channel 2317 and immobilize the rod 2315 relative to thehousing 2305.

As mentioned, the region 2330 of the housing is configured to enable afirst region of the housing 2305 to move relative to a second region ofthe housing 2305. The region 2330 enables the region of the housing thatis locked to the rod 2315 to move relative to the region of the housingthat is locked to the screw 2310. In this manner, the region 2330permits the rod 2315 to move relative to the screw 2310 while both therod and screw are immobilized relative to the housing 2305.

The region 2330 can be configured in various manners so as to permitsuch movement. In the illustrated embodiment, the region 2330 has apleated or corrugated configuration that permits the region 2330 toelastically flex or deform such that the segment of the housing 2305above the region 2330 can move relative to the segment below the region2330. It should be appreciated that the region 2330 can be configured invarious manners so as to permit such movement. Moreover, the region 2330can be configured to resist movement and to return to a defaultorientation after a load that caused the movement has dissipated.

FIG. 26 shows another embodiment of the assembly wherein thearticulating region 2330 is surrounded or covered by a sleeve 2605. FIG.27 shows a cross-sectional view of the assembly of FIG. 26. The sleeve2605 is an annular device that fits around the perimeter of the housing2305 so as to cover the region 2330. The sleeve 2605 can be a membranethat forms a sealed space that prevents migration of any wear debristhat may develop. The sleeve 2605 also serves as a barrier against theintrusion of connective tissue and the sealed space may contain alubricant to reduce friction. A pair of attachment rings 2610 can beused to secure the sleeve 2605 to the housing 2305. While not explicitlyillustrated on the other embodiments, this modification can be equallyadapted to them. Further, it should be appreciated that before lockingthe assembly, the surgeon can freely adjust the orientation of the screwrelative to housing without influencing the assembly's neutral positionor pre-loading the screw and bone construct.

FIGS. 28A-28B show side and cross-sectional views of an additionalembodiment of a bone screw 3800. The bone screw 3800 includes a head3810 and a shank 3812 that extends from the head. The screw 3800includes contains a movable intermediate segment 3815 between the screwshank 3812 that engages the bone and the screw head 3810 that lieswithin the housing of a screw assembly. While the remainder of theassembly is not depicted, the remainder can be substantially equal tothe housing/rod assembly shown in FIGS. 1 & 2. Alternatively, any screwassembly design that utilizes a rod and bone screw feature may be used.These devices are quite numerous and current art illustrates manyvariations of these assemblies.

The segment 3815 is configured to enable a first region of the screw3800 (such as the head 3810) to move relative to a second region of thescrew 2800 (such as the shank 3812). The segment 3815 enables the regionof the screw that is that is locked to the housing and rod to moverelative to the region of the housing that is locked to bone. In thismanner, the segment 3815 permits the rod to move relative to the screwwhile both the rod is immobilized relative to the housing.

The segment 3815 can be configured in various manners so as to permitsuch movement. In the illustrated embodiment, the segment 3815 has apleated or corrugated configuration that permits the segment 3815 toelastically flex or deform such that the head 3810 above the segment3815 can move relative to the shank 3812 below the segment 3815. Itshould be appreciated that the segment 3815 can be configured in variousmanners so as to permit such movement. In this device, a dynamic screwassembly is created by providing movement within the bone screw itself.As in the embodiment shown in FIGS. 26 and 27, a flexible sleeve ormembrane may be used to surround the segment 3815 of movablearticulation.

FIG. 29 shows yet another embodiment of the dynamic bone screw assembly.FIG. 30 shows an exploded view of the assembly of FIG. 29. In thisembodiment, the head of the screw is positioned within an inner housingmember in which the head can rotate in a ball and socket manner. Theinner housing member can be immobilized relative to the housing tofixedly attach the screw to the housing. However, the head of the screwcan rotate within the inner housing member to permit some movementbetween the screw and the housing. In addition, the head can becompletely immobilized within the inner housing.

With reference to FIGS. 29 and 30, the bone screw assembly 2800 includesan outer housing 2805, a bone screw 2810, and a rod 2815. A locking nut2820 can be threaded into the housing 2805 to provide a downward forceonto the rod 2815 and immobilize the rod relative to the housing 2805and the inner housing (2910 a & b). As best shown in FIG. 30, the bonescrew 2810 has a head 2905 that can be positioned within inner housingmembers 2910 a and 2910 b. While not shown, half members 2910 a & b arejoined to form the assembled inner housing member using threaded screws,ratchets, clips, adhesives, or any other technique for segment assembly.A saddle 2915 is positioned within the housing 2805 below the rod 2815and above the inner housing members 2910 in the assembled device.

FIG. 31 shows a cross-sectional view of the assembly of FIG. 29. Thehead 2905 of the screw 2910 is positioned within the inner housingmembers 2910, which collectively form a socket for the head 2905. Thesocket contains a space 3005 that is positioned, for example, above thehead 2905. The saddle 2915 is positioned directly above the innerhousing 2910 assembly and below the rod 2815.

The locking nut 2820 is advanced toward the rod 2815 to tightly pressthe rod 2815 against the upper edge of the saddle 2915. This also causesthe saddle 2915 to press downward against the inner housing members 2910and force the inner housing members 2910 against a seat in the housing2805, which causes rigid immobilization of the rod 2815, housing 2805,and inner housing members 2910 relative to one another. However, thehead 2905 of the bone screw 2810 is movable within the inner aspect ofthe inner housing members 2910 to produce the dynamic aspect of theassembly. That is, the head 2905 of the screw 2810 can rotatably movewithin the socket formed by the inner housing members 2910.

The space 3005 within the inner housing member 2910 can contain amaterial or structure that resists movement of the head 2905 of the bonescrew 2810 relative to the inner aspect of the inner housing members210. The material or structure within the space 3005 can be, forexample, an elastic material(s), fluids, spring device(s), magnets orany other appropriate materials/devices that will resist movement of thehead of bone screw relative to the inner aspect of the inner housingmembers. When the screw head is moved out of a predetermined position inthe inner housing members, the material/device within space 3005 willapply a force to the head of screw and resist any bone screw movementaway from the neutral position. With movement, the assembly would returnthe screw and the attached bone to the neutral position once thedeflecting force has dissipated. Further, before locking the assemblywith the locking nut 2820, the surgeon can freely adjust the orientationof the screw relative to housing without influencing the assembly'sneutral position or pre-loading the screw and bone construct.

FIG. 32 shows another embodiment of a dynamic bone screw assembly. FIG.33 shows the bone screw assembly of FIG. 31 in an exploded state. Thebone screw assembly 3100 includes a housing 3105, a bone screw 3110 thatfits through a bore in the housing 3105, and a rod 3115. The rod 3115lockingly engages a pair of locking members 3120. A pair of rotationalmember 3125 (FIG. 33) fit over the head 3130 and within the lockingmembers 3120, as shown in the cross-sectional view of FIG. 34. Thus,when assembled, the rotational members 3125 are interposed between thehead 3130 of the screw 3110 and the inner aspect of the locking members3120. While illustrated as using screws, the rotational members 3130 canbe attached to one another in various manners, such as using threadedscrews, ratchets, clips, adhesives, or any other technique for segmentassembly.

FIG. 34 shows a cross-sectional view of the assembly of FIG. 32. Thelocking members 3120 can lock to the housing 3105 the rod 3115 using aMorse taper configuration. When the locking members 3120 are presseddownward into the housing 3105 by the rod 3115, the two locking members3120 are forced inward toward the rod 3115 to immobilize the rod 3115therebetween. With the assembly in the locked configuration, the outersurfaces of the locking members 3120 tightly fit within the innersurface of the housing 3105. The individual segments of the lockingmembers 3120 are forced inward and immobilize the rod 3115 and therotational members 3125 relative to one another. In this way, theassembly serves to lock the rod 3115 relative to the bone screw 3110.

Although a Morse taper locking mechanism provides a powerfulimmobilization, it may be loosened with only a modest backout of thelocking members 3120 relative to the housing 3105. This may be preventedby the addition of a ratchet locker, wedge locker,protrusion/indentation locker, or any other locking mechanism to preventbackout and/or loosening of the Morse taper.

With reference to the cross-sectional view of FIG. 34, a space 3305 islocated between an upper surface of the screw head 3130 and an innersurface of the assembled rotational members 3125. The space 3305 cancontain deformable material, spring device(s), magnet(s), or any othermaterial or device that can resist movement of the upper surface of thehead 3130 relative to the inner surface of the assembled rotationalmembers 3125. When the assembly is in the unlocked state, the screw 3110and the rotational members 3125 are freely movable within the lockingmembers 3120.

When the assembly is locked, the rotational members 3125 are immobilizedrelative to the locking members 3120, but the screw head 3130 can stillrotate within the rotational members 3125. The material or device withinthe space 3305 applies a force to the screw head 3130 and resistsmovement of the screw head. In this manner, the screw resists adeflecting force but can also move within the rotational members 3120 ifthe force is of a sufficient magnitude. When the force has dissipated,the screw returns to the neutral position. FIG. 35 shows how the screw130 can rotate to various positions. Note that before locking theassembly, the surgeon can freely adjust the orientation of the screwrelative to housing without influencing the assembly's neutral positionor pre-loading the screw and bone construct. If desired, the head of thescrew can be completely immobilized within the rotational members 3125.

FIG. 36 shows the screw assembly of FIG. 32 placed into the pedicleportion of a vertebra. Four screw assemblies 3100 have been placed intothe two vertebras V1 and V2. Inter-connecting rods 3115 are used toconnect the two screw assemblies 3100 on each side of the midline andall of the screw assemblies 3100 are then locked. The screw assembliespermit dynamic fixation of the vertebral bodies.

FIGS. 37A-C show additional embodiments of bone screw assemblies whereina rod can be immobilized using a Morse taper assembly, such as wasdescribed above. One or more of the components of the assembly can bemanufactured of a deformable material that permits some relativemovement between the screw and the rod when a force of sufficientmagnitude is applied thereto. In this manner, a dynamic screw assemblycan be achieved.

FIG. 37A shows an exploded view of an embodiment of a dynamic bone screwassembly 3700. FIG. 37B shows a cross-sectional view of the bone screwassembly of FIG. 37A. The bone screw assembly 3700 includes a housing3705, a bone screw 3710, a rod 3715, a locking nut 3720, and an innerhousing 3725. An upper member 3730 is positioned above the rod 3715 andis rotatably attached to an underside of the locking nut 3720. As shownin the cross-sectional view of FIG. 37C, the rod 3715 is compressedbetween the inner housing 3725 and the upper member 3730 when thelocking nut 3720 is advanced downward into the housing 3705. The housing3705 has a bore that is large enough to receive a shank portion of thescrew 3710, but not large enough that the head of the screw 3710 canpass through the bore.

The upper member 3730 and the inner housing 3725 can be manufactured ofa deformable material or a shape-memory memory material to permit thescrew 3710 to be rotated out of the neutral position. The material isdeformed when the screw 3710 is moved out of the neutral position, butprovides a force on the screw the urges the screw back towards theneutral position.

FIG. 37C shows an exploded view of another embodiment of a dynamic bonescrew assembly 3750. FIG. 37D shows a cross-sectional view of the bonescrew assembly of FIG. 37C. The bone screw assembly 3750 includes ahousing 3755, a bone screw 3760, a rod 3765. The housing includes aninner sleeve 3770 that surrounds a deformable inner housing 3775. Therod 3765 can be advanced downward to clamp the rod and 3765 and the headof the screw 3760 within the inner housing 3775 using a Morse typeconfiguration. The inner housing 3775 can be manufactured of adeformable material or a shape-memory memory material to permit thescrew 3760 to be rotated out of the neutral position. The material isdeformed when the screw 3760 is moved out of the neutral position, butprovides a force on the screw the urges the screw back towards theneutral position.

The dynamic bone screw assemblies of the type described herein provideone or more movable elements between the screw shaft and the rod. Inthis way, the rod can be immobilized relative to one movable elementwhile the second movable element provides continued movement of thescrew shaft relative to the rod. Regardless of the particular lockingmechanism, such a feature can be used to provide dynamic stabilizationof the bony elements.

FIG. 38 show various schematic embodiments of bone screw assemblies thatemploy multiple moving surfaces or elements. A generic locking mechanismis shown in conjunction with a rod R, a bone screw S and two elements E1and E2 that can move relative to one another. The rod R can be locked orimmobilized relative to element E1, while the screw S can be locked orimmobilized relative to element E2. In embodiments A, B and C, theelement E2 is movably disposed within element E1 and in embodiments C,D, and E, the element E1 is movably disposed within element E2. Inembodiment B, the element E2 extends downwardly from element E1 todisplace the screw S from element E1. Embodiment C also displaces thescrew S, but the element E2 has a portion that is positioned inside thescrew S. In embodiment D, the element E1 is disposed within element E2,and element E2 is disposed within the screw S. In embodiment E, theelement E1 and the screw are both disposed within the element E2.Finally, in embodiment F, the element E1 is disposed within element E2,which is disposed within the screw S. One of ordinary skill in the artcan add additional surfaces or elements to achieve further embodimentsof this invention and it is understood that these would fall within thescope of this application.

Although embodiments of various methods and devices are described hereinin detail with reference to certain versions, it should be appreciatedthat other versions, embodiments, methods of use, and combinationsthereof are also possible. Further, the design elements andmodifications disclosed in the application that permit dynamic movementbetween rod and bone screw may be alternatively applied to any screwassembly that utilizes a rod and bone screw feature. These devices arequite numerous and current art illustrates many variations of theseassemblies that are presently configured for rigid fixation alone.(e.g., U.S. Pat. Nos. 5,810,819; 6,139,549; 6,371,957; 6,379,357;6,478,798; 6,565,565; 6,610,063 and many others disclose variations ofthese devices.) Therefore the spirit and scope of the appended claimsshould not be limited to the description of the embodiments containedherein.

1. A bone fixation assembly, comprising: an elongate rod; a bonefixation member adapted to be secured to a spinal segment; a housingassembly having a first portion adapted to be removably attached to therod in a manner that immobilizes the rod relative to the first portion,the housing assembly also having a second portion adapted to beremovably attached to the fixation member to place the rod and thefixation member in proximity; wherein at least a portion of the bonefixation assembly can articulate while the rod is immobilized relativeto the housing assembly to permit the fixation member to move from aninitial orientation to a different orientation relative to the rod inresponse to application of a load on the bone fixation assembly, andwherein the fixation member is automatically urged toward the initialconfiguration when the load is removed.
 2. A bone fixation assembly asin claim 1, wherein the housing assembly includes an inner housinghaving a slot that receives the rod and an outer housing having a seatthat receives a head of the bone fixation member, and wherein the innerhousing mounts within the outer housing such that the inner housing canmove relative to the outer housing to permit the fixation member to movefrom the initial orientation to the second orientation.
 3. A bonefixation assembly as in claim 2, wherein at least one space ispositioned between the outer housing and the inner housing, and whereina material or structure is positioned within the space, the material orstructure adapted to resist relative movement between the inner housingand the outer housing.
 4. A bone fixation assembly as in claim 2,further comprising: a first locking member adapted to immobilize the rodrelative to the inner housing; a second locking member adapted toimmobilize the fixation member relative to the outer housing; and athird locking member that immobilizes the inner housing relative to theouter housing.
 5. A bone fixation assembly as in claim 1, wherein thehousing assembly or the fixation member includes an articulation regionthat can articulate to permit the first portion of the, housing assemblyto move relative to the second portion.
 6. A bone fixation assembly asin claim 5, wherein the articulation region is corrugated.
 7. A bonefixation assembly as in claim 5, further comprising a sheath thatsurrounds the articulation region.
 8. A bone fixation assembly as inclaim 5, wherein the articulation region is an elastically deformablematerial.
 9. A bone fixation assembly as in claim 8, wherein thearticulation region is a shape memory material.
 10. A bone fixationassembly as in claim 1, wherein the housing assembly includes an outerhousing with slot that receives the rod, the outer housing furtherincluding a seat, and wherein the housing assembly further comprises aninner housing positioned in the seat, the inner housing adapted to beimmobilized relative to the outer housing, wherein the inner housingdefines a socket in which a head of the bone fixation member is movablypositioned.
 11. A bone fixation assembly as in claim 10, wherein amaterial or structure is positioned within the socket to resist movementof the head of the fixation member within the socket.
 12. A bonefixation assembly as in claim 1, wherein the housing assembly includes aslot adapted to receive the rod and a seat adapted to receive a head ofthe fixation member below the rod, wherein a material or structure ispositioned within the seat to resist movement of the head of thefixation member within the seat.
 13. A bone fixation assembly as inclaim 1, wherein at least a portion of the bone fixation assembly is atleast partially formed of a shape-memory material, said shape-memorymaterial exhibiting pseudoelastic characteristics at about bodytemperature.
 14. A bone fixation assembly as in claim 13, wherein atleast a portion of the housing assembly is formed of the shape-memorymaterial.
 15. A bone fixation assembly as in claim 13, wherein at leasta portion of the fixation member is formed of the shape-memory material.16. A bone fixation assembly as in claim 1, wherein the housing assemblyincludes an outer housing and at least one locking member positionedwithin the outer housing and adapted to receive the rod, the housingassembly further comprising a rotational member rotatably positionedwithin the locking member and the outer housing, the rotational memberforming a socket in which a head of the fixation member is rotatablypositioned; wherein the rod can be pressed downward into the lockingmember to immobilize the rod in the locking member and the lockingmember in the housing and further immobilize the rotational memberrelative to the locking member while permitting rotatable movement ofthe head of the fixation member within the socket, and wherein amaterial or structure within the socket applies a force to the head ofthe fixation member upon rotational movement of the fixation member toresist movement of the fixation member out of the initial orientation.17. A bone fixation assembly as in claim 1, wherein the bone fixationmember comprises a bone screw.
 18. A method of stabilizing the spine,comprising: providing a first fixation assembly including a firsthousing, a first bone screw, and a rod; securing the first fixationassembly to a first spine segment such that the first bone screw isfixated to the first spine segment; and locking the first fixationassembly such that the rod is immobilized relative to the first housingwhile the first bone screw can move from an initial orientation to adifferent orientation relative to the rod in response to application ofa load on the first bone fixation assembly, and wherein the first bonescrew is automatically urged toward the initial configuration when theload is removed.
 19. A method as in claim 18, further comprising:providing a second fixation assembly including a second housing and arod; securing the second fixation assembly to a second spine segmentsuch that the second bone screw is fixated to the second spine segment;and securing the second fixation assembly to the rod, wherein the rod isimmobilized relative to the second bone screw such that the rod isfixedly cantilevered from the second bone screw assembly.